1. Field of the Invention
The present invention relates to an optical measurement device which uses light beams which are incident upon and transmitted through or reflected by an object, especially a light scattering body, to optically measure the object.
2. Description of the Related Art
The major difficulty of optical measurement of highly scattering media such as, for example, biological systems including human tissues, lies in how to extract the signal light propagated along a traceable optical path from the other transmitted of reflected lights which have been multiply scattered inside the scattering medium. One of the several detection methods is to incident onto the scattering sample an ultra-shot laser pulse having a pulse width of a few picoseconds(picosecond=10.sup.-12 sec.), and detect the temporal profile of the transmitted laser pulse with an ultra-high speed Streak camera. In this time-resolved detection method, the early arriving photons at the Streak camera appear to travel the shortest distance inside the scattering sample so that they can be regarded as the least-scattered or apparently unscattered light components (refer to, for example, Optics Letters, vol.15, 1179 (1990) by S. Anderson-Engeles, R. Berg, S. Svanberg, O. Jarlman).
On the other hand, as one of spatially resolved methods, by noting a loss of directional property for the scattered light, there is proposed a method in which an apparently unscattered light component keeping its directional property is detected in an optical heterodyne detection method having a superior direction selectivity which is known as an antenna property (refer to, for example, Electronics Letters, Vol. 26, 700 (1990) by M. Toida, M. Kondo, T. Ichimura, H. Inaba).
FIG. 18 is a principle diagram of the optical heterodyne detection method.
A coherent laser light beam 11a emitted from a laser light source 11 is collimated by using lenses 12 to form a light beam with a predetermined beam diameter, and split by a beam splitter 13 into a signal light 11b and a reference light 11c. The signal light 11b separated from the reference light 11c by the beam splitter 13 is incident upon a sample 100 which is constituted of, for example, a scattering body. The signal light transmitted through the sample 100 is passed via a beam splitter 14 onto a photo detector 15.
On the other hand, the reference light 11c separated from the signal light 11b by the beam splitter 13 is reflected by a mirror 16, then frequency-shifted by an AOM (acousto-optic modulator) or another frequency shifter 17, further reflected by a mirror 18, superimposed on the signal light by the beam splitter 14, and mixed with the signal light on the surface of the photo detector 15. On the photo detector 15 produced is an interference light constituted of the signal light and the reference light which are caused to differ in frequencies from each other by the action of the frequency shifter 17. The output from photo detector 15 consists of a heterodyne signal 15a with a frequency which equals to the frequency difference between the signal light and the reference light.
Optical measurement based on the optical heterodyne detection method detects, intrinsically, in addition to an unscattered light component that advances along the optical axis (on-axis), a near-axis forward scattered light component which, while being multiply scattered inside the sample, retains partially the temporal coherence of the incident light and emerges from the sample in the same direction of the unscattered light. Such a near-axis forward scattered light component exhibits a relatively smaller extinction coefficient as compared with that of the unscattered light, as it has been recently pointed out (refer to Applied Physics B, Vol. 63, 249 (1996) by K. P. Chan, M. Yamada, H. Inaba).
FIG. 19 is given to illustrate an issue in the application of optical heterodyne detection to optical measurement of scattering medium. The performance of heterodyne detection can be adversely affected by light scattering on the exit surface 100a of the sample 100.
The unscattered component and the near-axis forward scattered light component which have been passed though the scattering sample 100 carry information on the internal structure of the sample 100. However, when the exit surface 100a of the sample 100 is not optically flat, i.e., when there exist irregularities in the order of wavelength, light scattering on the exit surface 100a causes the transmitted light spread out rapidly, as it can be explained by the light scattering theory (refer to, for example, "Wave Propagation and Scattering in Random Media", authored by A. Ishimaru and published by Academic Press (1978)). Specifically, following the scattering theory, an outgoing light is scattered and spread out as shown in FIG. 19, for example. In the conventional optical heterodyne method shown in FIG. 18, since a field of view in the optical heterodyne is narrowly restricted (.theta.=.lambda./D, in which .lambda. denotes a wavelength, while D denotes a size of a photo detector), only a part of signal lights emitted from the sample 100 can be detected. Specifically, detected are the signal lights which advance straight in the narrow field of view, i.e., within a remarkably small angle defined by .theta.=.lambda./D. As a result, the amount of signal light that can be effectively detected by using the heterodyne detection method shown in FIG. 19 can be substantially reduced. Meanwhile, it has been pointed out that in the conventional optical heterodyne method, the light signals spatially distorted by the aforementioned scattering cannot be effectively detected (refer to Optics Letters, Vol. 17, 1237 (1992) by K. P. Chan, D. K. Killinger).
The aforementioned surface scattering not only reduces the amount of signals that can be detected by heterodyne detection, but also induces laser speckle noises. Optical heterodyne detection is based on the interference between the signal light and the reference light. Yet the interference between scattered lights with random phases results in laser speckles, which cause significant fluctuations in the heterodyne signal intensity. Laser speckle is a well known phenomenon (refer to, for example, "Laser Speckle and Related Phenomena", edited by J. C. Dainty and published by Springer-Verlag Co. (New York, 1975)).
FIGS. 20 and 21 are measurement results showing the fluctuations in heterodyne signal intensities which are caused by speckle noises, by using the optical heterodyne detection method described in FIG. 19.
In FIGS. 20 and 21, laser beams each having a beam diameter of 1 mm and a wavelength of 1.064 .mu.m are incident onto a 5 mm-thick slab of potato (FIG. 20) and a 5 mm-thick slab of lean pork (FIG. 21), and lights transmitted though the respective samples are detected by a heterodyne detector. FIGS. 20 and 21 have abscissas which represent time in the unit of seconds and minutes, respectively.
As seen from FIGS. 20 and 21, the time periods of heterodyne signal intensity fluctuations can significantly vary with types of scattering samples. It is well known that in the presence of speckle noises the statistics of heterodyne signal intensity follows the Rayleigh distribution with a standard deviation .sigma.=1. Meanwhile, it is also known that speckle noises can be averaged out by averaging the heterodyne detection signals. However, as shown in the example of FIG. 21, when in the sample about two minutes are required for the speckle pattern to change to the next statistically random state, it takes a considerably long period of time to average out the independent speckles by this speckle averaging method.
The reduced heterodyne detection efficiency and speckle noises described are common in heterodyne detection of signal light reflected from the sample as well. Although optical heterodyne detection has been chosen as a typical example, it is understood that other measurement methods based on optical interference may share these problems in common.